Our understanding of reaction mechanisms in biological systems at the molecular level is presently based almost entirely on knowledge of static, chemically- or physically-trapped structures. However, the dynamic aspects of changes in structure are critical during processes such as catalysis, ligand binding and release, protein (re)folding and signal transduction. These changes can be extremely rapid and the lifetime of structural intermediates correspondingly small, from seconds to picoseconds and even shorter. The changes can also be small in spatial extent, which demands high crystallographic resolution and accurate imeasurement of small changes in X-ray structure amplitudes. We have successfully conducted high resolution, time-resolved crystallographic experiments with approximately 100 picosecond time resolution on light-sensitive systems, identified the structures of short-lived intermediates and characterized the overall mechanism. We will apply time-resolved crystallography to elucidate the mechanism of light-dependent signal transduction in selected, water-soluble photoreceptors, at the atomic level. These contain one or more sensor domains that bind a small, non-protein chromophore, together with an effector domain. After absorption of a photon by the chromophore, prompt electronic and structural changes in the chromophore itself cause structural changes in the sensor domain and ultimately in the effector domain. These changes in structure modulate the activity of the effector domain; an otherwise light-inert biochemical activity such as phosphorylation becomes light-sensitive. The photoreceptor sensor domains bind chemically diverse chromophores such as FMN, FAD, p-coumaric acid or heme and are linked to structurally diverse effector domains with; e.g., serine/threonine or histidine kinase activity. Bacterial and plant photoreceptor systems will be studied, as isolated sensor and effector domains and in naturally-occurring, covalent and noncovalent complexes.